[Show abstract][Hide abstract]ABSTRACT: The photochemistry of N2 and CH4 in the atmosphere of Titan leads to a very rich chemistry which is not well understood. The aim of our study is to improve our understanding of the production of nitrogen compounds and to predict the abundances of those with high molar mass with better accuracy. We have made a careful investigation of the neutral nitrogen photochemistry to improve current chemical schemes including the most abundant species and the most efficient reactions. We also studied the propagation of uncertainties on rate constants in our model and determined the key reactions from a global sensitivity analysis. Our photochemical model contains 124 species, 60 of which are nitrogen containing compounds, and 1141 reactions. Our results are in reasonable agreement with Cassini/INMS data in the higher atmosphere but our model overestimates the mole fractions of several nitriles in the lower stratosphere. New species such as CH3C3N and C3H7CN could be relatively abundant in Titan's atmosphere. Uncertainties on some nitrogen compounds are important and further studies of the key reactions that we have identified are needed to improve the predictivity of photochemical models. Meridional transport is expected to be an efficient process to govern the abundances of several nitriles in the lower stratosphere.

[Show abstract][Hide abstract]ABSTRACT: Context. The increased sensitivity and high spectral resolution of millimeter
telescopes allow the detection of an increasing number of isotopically
substituted molecules in the interstellar medium. The 14N/ 15N ratio is
difficult to measure directly for carbon containing molecules. Aims. We want to
check the underlying hypothesis that the 13C/ 12C ratio of nitriles and
isonitriles is equal to the elemental value via a chemical time dependent gas
phase chemical model. Methods. We have built a chemical network containing D,
13C and 15N molecular species after a careful check of the possible
fractionation reactions at work in the gas phase. Results. Model results
obtained for 2 different physical conditions corresponding respectively to a
moderately dense cloud in an early evolutionary stage and a dense depleted
pre-stellar core tend to show that ammonia and its singly deuterated form are
somewhat enriched in 15N, in agreement with observations. The 14N/ 15N ratio in
N2H+ is found to be close to the elemental value, in contrast to previous
models which obtain a significant enrichment, as we found that the
fractionation reaction between 15N and N2H+ has a barrier in the entrance
channel. The large values of the N2H+/15NNH+ and N2H+/ N15NH+ ratios derived in
L1544 cannot be reproduced in our model. Finally we find that nitriles and
isonitriles are in fact significantly depleted in 13C, questioning previous
interpretations of observed C15N, HC15N and H15NC abundances from 13C
containing isotopologues.

[Show abstract][Hide abstract]ABSTRACT: Recent observations have revealed the existence of Complex Organic Molecules
(COMs) in cold dense cores and prestellar cores. The presence of these
molecules in such cold conditions is not well understood and remains a matter
of debate since the previously proposed "warm- up" scenario cannot explain
these observations. In this article, we study the effect of Eley- Rideal and
complex induced reaction mechanisms of gas-phase carbon atoms with the main ice
components of dust grains on the formation of COMs in cold and dense regions.
Based on recent experiments we use a low value for the chemical desorption
efficiency (which was previously invoked to explain the observed COM
abundances). We show that our introduced mechanisms are efficient enough to
produce a large amount of complex organic molecules in the gas-phase at
temperatures as low as 10K.

[Show abstract][Hide abstract]ABSTRACT: The coupled photochemistry of N2, CH4 and oxygen species in the atmosphere of Titan leads to a very rich chemistry which is not well understood. The aim of our study is to improve our understanding of the production of hydrocarbons and nitrogen compounds and to understand the origin of oxygen compounds in the atmosphere. As a first step, we have made a detailed investigation of neutral photochemistry of these species. New species, reactions and photolysis processes have been added from an extensive bibliography and theoretical works. We have also investigated the possible input of sulphur species in the atmosphere. Through the use of a Monte Carlo-based uncertainty propagation study and global sensitivity analysis, we have identified the key reactions that should be studied in priority to improve photochemical models of Titan's atmosphere.

[Show abstract][Hide abstract]ABSTRACT: We studied the hypothesis that micrometeorites and Enceladus' plume activity could carry sulfur-bearing species into the upper atmosphere of Titan, in a similar manner to oxygen-bearing species.
We have developed a detailed photochemical model of sulfur compounds in the atmosphere of Titan that couples hydrocarbon, nitrogen, oxygen and sulfur chemistries.
Photochemical processes produce mainly CS and H2CS in the upper atmosphere of Titan and C3S, H2S and CH3SH in the lower atmosphere. Mole fractions of these compounds depend significantly on the source of sulfur species.
A possible future detection of CS (or the determination of a low upper limit) could be used to discriminate between the two scenarios for the origin of sulfur species, which then could help to discriminate the various scenarios for the origin of H2O, CO and CO2 in the stratosphere of Titan.

[Show abstract][Hide abstract]ABSTRACT: Rate constants for the N((4)S) + C2((1)Σg(+)) reaction have been measured in a continuous supersonic flow reactor over the range 57 K ≤ T ≤ 296 K by the relative rate technique employing the N((4)S) + OH(X(2)Π) → H((2)S) + NO(X(2)Π) reaction as a reference. Excess concentrations of atomic nitrogen were produced by the microwave discharge method and C2 and OH radicals were created by the in situ pulsed laser photolysis of precursor molecules C2Br4 and H2O2 respectively. In parallel, quantum dynamics calculations were performed based on an accurate global potential energy surfaces for the three lowest lying quartet states of the C2N molecule. The 1(4)A'' potential energy surface is barrierless, having two deep potential wells corresponding to the NCC and CNC intermediates. Both the experimental and theoretical work show that the rate constant decreases to low temperature, although the experimentally measured values fall more rapidly than the theoretical ones except at the lowest temperatures. Astrochemical simulations indicate that this reaction could be the dominant source of CN in dense interstellar clouds.

[Show abstract][Hide abstract]ABSTRACT: We review the reactions involving HCN and HNC in dark molecular clouds to
elucidate new chemical sources and sinks of these isomers. We find that the
most important reactions for the HCN-HNC system are Dissociative Recombination
(DR) reactions of HCNH+ (HCNH+ + e-), the ionic CN + H3+, HCN + C+, HCN and HNC
reactions with H+/He+/H3+/H3O+/HCO+, the N + CH2 reaction and two new
reactions: H + CCN and C + HNC. We test the effect of the new rate constants
and branching ratios on the predictions of gas-grain chemical models for dark
cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio
significantly above one as long as the carbon atom abundance remains high.
However, the reaction of HCN with H3+ followed by DR of HCNH+ acts to isomerize
HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio
close to or slightly greater than 1. This agrees well with observations in
TMC-1 and L134N taking into consideration the overestimation of HNC abundances
through the use of the same rotational excitation rate constants for HNC as for
HCN in many radiative transfer models.

[Show abstract][Hide abstract]ABSTRACT: Rate constants for the C(3P) + CH3OH reaction have been measured in a
continuous supersonic flow reactor over the range 50 K to 296 K. C(3P) was
created by the in-situ pulsed laser photolysis of CBr4, a multiphoton process
which also produced some C(1D), allowing us to investigate simultaneously the
low temperature kinetics of the C(1D) + CH3OH reaction. C(1D) atoms were
followed by an indirect chemiluminescent tracer method in the presence of
excess CH3OH. C(3P) atoms were detected by the same chemiluminescence technique
and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF).
Secondary measurements of product H(2S) atom formation have been undertaken
allowing absolute H atom yields to be obtained by comparison with a suitable
reference reaction. In parallel, statistical calculations have been performed
based on ab-initio calculations of the complexes, adducts and transition states
(TSs) relevant to the title reaction. By comparison with the experimental H
atom yields, the preferred reaction pathways could be determined, placing
important constraints on the statistical calculations. The experimental and
theoretical work are in excellent agreement, predicting a negative temperature
dependence of the rate constant increasing from 2.2 x 10-11 cm3 molecule-1 s-1
at 296 K to 20.0 x 10-11 cm3 molecule-1 s-1 at 50 K. CH3 and HCO are found to
be the major products under our experimental conditions. As this reaction is
not considered in current astrochemical networks, its influence on interstellar
methanol abundances is tested using a model of dense interstellar clouds.

[Show abstract][Hide abstract]ABSTRACT: Analysis of recent detections of water by Herschel/HIFI-PACS and Cassini/CIRS suggest for a steep gradient of the water profile in the lower stratosphere of Titan's atmosphere (Cottini2012, Moreno2012). This result provides a good opportunity to better understand the origin of oxygen compounds. However, the current photochemical models use an incomplete oxygen chemical scheme. In the present work, we improve the photochemistry of oxygen and introduce in particular a coupling between hydrocarbon, oxygen and nitrogen chemistries. Through the use of several different scenarios, we show that some oxygen compound abundances are sensitive to the nature of oxygen atoms (O+, OH and H2O) and the source of the flux (micrometeorites ablation or Enceladus' plume activity). Our model also predicts the presence of new and as yet undetected compounds such as NO (nitric oxide), HNO (nitrosyl hydride), HNCO (isocyanic acid) and N2O (nitrous oxide). Their future putative detection will give valuable constraints to discriminate between the different hypotheses for the nature and the source of oxygen compounds in the atmosphere of Titan. Through the use of a Monte Carlo-based uncertainty propagation study and global sensitivity analysis, we identify the key reactions that should be studied in priority to improve coupled photochemical models of Titan's atmosphere.

[Show abstract][Hide abstract]ABSTRACT: We review the reactions between carbon chain molecules and radicals, namely
Cn, CnH, CnH2, C2n+1O, CnN, HC2n+1N, with C, N and O atoms. Rate constants and
branching ratios for these processes have been re-evaluated using experimental
and theoretical literature data. In total 8 new species have been introduced,
41 new reactions have been proposed and 122 rate coefficients from
kida.uva.2011 (Wakelam et al. 2012) have been modified. We test the effect of
the new rate constants and branching ratios on the predictions of gas-grain
chemical models for dark cloud conditions using two different C/O elemental
ratios. We show that the new rate constants produce large differences in the
predicted abundances of carbon chains since the formation of long chains is
less effective. The general agreement between the model predictions and
observed abundances in the dark cloud TMC-1 (CP) is improved by the new network
and we find that C/O ratios of 0.7 and 0.95 both produce a similar agreement
for different times. The general agreement for L134N (N) is not significantly
changed. The current work specifically highlights the importance of O + CnH and
N + CnH reactions. As there are very few experimental or theoretical data for
the rate constants of these reactions we highlight the need for experimental
studies of the O + CnH and N + CnH reactions, particularly at low temperature.

[Show abstract][Hide abstract]ABSTRACT: Predictions of astrochemical models depend strongly on the reaction rate
coefficients used in the simulations. We reviewed a number of key reactions for
the chemistry of nitrogen-bearing species in the dense interstellar medium and
proposed new reaction rate coefficients for those reactions. The details of the
reviews are given in the form of a datasheet associated with each reaction. The
new recommended rate coefficients are given with an uncertainty and a
temperature range of validity and will be included in KIDA
(http://kida.obs.u-bordeaux1.fr).

[Show abstract][Hide abstract]ABSTRACT: Water is an important reservoir species for oxygen in interstellar space and
plays a key role in the physics of star formation through cooling by
far-infrared emission. Whilst water vapour is present at high abundances in the
outflows of protostars, its contribution to the chemical evolution of these
regions is a minor one due to its limited low temperature reactivity in the
gas-phase. Here, we performed kinetic experiments on the barrierless CH + H2O
reaction in a supersonic flow reactor down to 50 K. The measured rate increases
rapidly below room temperature, confirming and extending the predictions of
earlier statistical calculations. The open product channels for this reaction
suggest that this process could be an important gas-phase route for
formaldehyde formation in protostellar envelopes.

[Show abstract][Hide abstract]ABSTRACT: Rate constants for the potentially important interstellar N((4)S) + CH(X(2)Πr) reaction have been measured in a continuous supersonic flow reactor over the range 56 K ≤ T ≤ 296 K using the relative rate technique employing both the N((4)S) + OH(X(2)Πi) and N((4)S) + CN(X(2)Σ(+)) reactions as references. Excess concentrations of atomic nitrogen were produced by the microwave discharge method upstream of the Laval nozzle and CH and OH radicals were created by the in situ pulsed laser photolysis of suitable precursor molecules. In parallel, quantum dynamics calculations of the title reaction have been performed based on accurate global potential energy surfaces for the 1(3)A' and 1(3)A'' states of HCN and HNC, brought about through a hierarchical construction scheme. Both adiabatic potential energy surfaces are barrierless, each one having two deep potential wells suggesting that this reaction is dominated by a complex-forming mechanism. The experimental and theoretical work are in excellent agreement, predicting a positive temperature dependence of the rate constant, in contrast to earlier experimental work at low temperature. The effects of the new low temperature rate constants on interstellar N2 formation are tested using a dense cloud model, yielding N2 abundances 10-20% lower than previously predicted.

[Show abstract][Hide abstract]ABSTRACT: The description of C3 hydrocarbon chemistry in current photochemical models of Titan’s atmosphere is found to be far from complete. We have carefully investigated the photochemistry involving C3Hp compounds in the atmosphere of Titan (considering both photolysis and neutral reactions), which considerably impacts the abundances of many other hydrocarbon species (including C2 compounds). Model results indicate that three species (C3, c–C3H2 and C3H3) could be abundant enough to be present in the Cassini/INMS data. Because the error bars on predicted C3-hydrocarbon abundances are considerably larger than those of the observational data, new experimental and theoretical studies targeting the measurement of low-temperature reaction rate constants and product branching ratios are required to reduce current model uncertainties. In particular, we highlight 30 “key reactions”, the uncertainty factors of which should be lowered to improve the quality of photochemical models involving C3Hp molecules.

[Show abstract][Hide abstract]ABSTRACT: The gas phase reaction of the hydroxyl radical with allene has been studied theoretically and experimentally in a continuous supersonic flow reactor over the range 50 ≤ T/K ≤ 224. This reaction has been found to exhibit a negative temperature dependence over the entire temperature range investigated, varying between (0.75 and 5.0) × 10(-11) cm(3) molecule(-1) s(-1). Product formation from the reaction of OH and OD radicals with allene (C(3)H(4)) has been investigated in a fast flow reactor through time-of-flight mass spectrometry, at pressures between 0.8 and 2.4 Torr. The branching ratios for adduct formation (C(3)H(4)OH) in this pressure range are found to be equal to 34 ± 16% and 48 ± 16% for the OH and OD + allene reactions, respectively, the only other channel being the formation of CH(3) or CH(2)D + H(2)CCO (ketene). Moreover, the rate constant for the OD + C(3)H(4) reaction is also found to be 1.4 times faster than the rate constant for the OH + C(3)H(4) reaction at 1.5 Torr and at 298 K. The experimental results and implications for atmospheric chemistry have been rationalized by quantum chemical and RRKM calculations.

[Show abstract][Hide abstract]ABSTRACT: Recent advances in Earth and satellite based observations of molecules in interstellar environments and in planetary atmospheres have highlighted the lack of information regarding many important gas-phase formation mechanisms involving neutral species at low temperatures. Whilst significant progress has been made towards a better understanding of radical-molecule reactions in these regions, the inherent difficulties involved in the investigation of reactions between two unstable radical species have hindered progress in this area. This perspective article provides a brief review of the most common techniques applied to study radical-radical reactions below room temperature, before outlining the developments in our laboratory that have allowed us to extend such measurements to temperatures relevant to astrochemical environments. These developments will be discussed with particular emphasis on our recent investigations of the reactions of atomic nitrogen with diatomic radicals.